I have learned through my years of experience that the only way to defeat chaos is.....TO IGNORE IT!>>17825822
About the sterilization thing. Most reports of this come from the imperials and we all know how truthful and accurate these guys are.
Anyways, I could imagine an Ethereal or a Commander who are not very fond of humanity ordering the sterilization of a resistant Gue'la population to lessen their threat.
However this action will go against Tau'va and the Tau Empire's mission of enlightening the Gue'las and redeeming them.
But remember this is war and not a simple war. It IS a war against the most brutal regime and genocidal force the galaxy ever known, The Imperium of man.
War against such a hateful foe would taint any saint and make him commit questionable things.
I'm not admitting that the Tau indeed sterilize the Gue'las, but if its true (Most unlikely) then there should be a reasonable justification for it.